A computer's components, including its processor, chipset, cache, memory, expansion cards and storage devices, communicate with each other over one or more “buses”. A “bus”, in general computer terms, is a channel over which information flows between two or more devices. A bus normally has access points, or places to which a device can connect to the bus. Once connected, devices on the bus can send to, and receive information from, other devices.
Today's personal computers tend to have at least four buses. Each bus is to some extent further removed from the processor; each one connects to the level above it.
The Processor Bus is the highest-level bus, and is used by the chipset to send information to and from the processor. The Cache Bus (sometimes called the backside bus) is used for accessing the system cache. The Memory Bus connects the memory subsystem to the chipset and the processor. In many systems, the processor and memory buses are the same, and are collectively referred to as the frontside bus or system bus.
The local I/O (input/output) bus connects peripherals to the memory, chipset, and processor. Video cards, disk storage devices, and network interface cards generally use this bus. The two most common local I/O buses are the VESA Local Bus (VLB) and the Peripheral Component Interconnect (PCI) bus. An Industry standard architecture (ISA) I/O Bus may also be used for slower peripherals, such as mice, modems, and low speed sound and networking devices.
The current generation of PCI bus is known as the PCI Express bus. This bus is a high-bandwidth serial bus, which maintains software compatibility with existing PCI devices.